1. Field of the Invention
The present invention relates to an improvement in a facing-targets-type sputtering apparatus configured such that a pair of facing targets are disposed a predetermined distance away from each other and such that a magnetic field extending between the targets from one target to the other is generated in such a manner as to surround a space provided between the facing targets (the space is hereinafter called a discharge space) to thereby confine plasma within the discharge space and to form a film on a substrate disposed at a position beside the discharge space under vacuum, as well as to an improvement in a sputtering method using the apparatus. Particularly, the invention relates to a facing-targets-type sputtering apparatus and method capable of adjusting a sputtering voltage over a wide range.
2. Description of the Related Art
A facing-targets-type sputtering method is disclosed in patent applications previously filed by the present inventors (Japanese Publication of Examined Patent Application (kokoku) Nos. S63-20303, S63-20304, and S62-14633). An apparatus used in the method is shown in FIG. 1. As shown in FIG. 1, the apparatus has the following basic configuration. Targets 110a and 110b are disposed a predetermined distance away from each other within a vacuum chamber vessel 10 having a chamber wall 11, thereby defining a discharge space 120 therebetween. Magnetic-field generation means 130a and 130b are disposed behind the corresponding targets 110a and 110b in order to generate a magnetic field which extends between the targets 110a and 110b from one target to the other and whose flux uniformly surrounds the discharge space 120. A substrate holder 21 disposed at a position beside the discharge space 120 holds a substrate 20 such that the substrate 20 faces the discharge space 120. Reference numerals 140a and 140b denote shields for protecting from sputtering portions of target units 100a and 100b other than the front surfaces of the targets 110a and 110b. 
After the vacuum chamber vessel 10 is evacuated through an evacuation port 30 by means of an unillustrated evacuation system, a sputtering gas, such as argon, is introduced into the vacuum chamber vessel 10 through a gas inlet 40 by means of an unillustrated gas introduction means. As shown in FIG. 1, a DC power supply 50 serving as a sputtering power supply supplies sputtering power to the apparatus while the shields 140a and 140b; i.e., the vacuum chamber vessel 10, serve as an anode (ground) and the targets 110a and 110b serve as a cathode. Thus, sputtering plasma is generated while being confined within the discharge space 120 by means of the magnetic field. The sputtering plasma effects sputtering of the targets 110a and 110b, thereby forming on the substrate 20 a thin film whose composition corresponds to that of the targets 110a and 110b. 
According to the method, since the magnetic field extends in the direction extending between the targets 110a and 110b; i.e., perpendicularly to the targets 110a and 110b, high-energy electrons are confined within the discharge space 120 so as to generate sputtering plasma. The sputtering plasma accelerates ionization of the sputtering gas, thereby increasing a sputtering rate and thus forming a film at high rate. In contrast to a typical conventional magnetron type sputtering method, in which a substrate is disposed in opposition to a target, the substrate 20 is disposed at a position beside the targets 110a and 110b. Accordingly, ions and electrons impinging on the substrate 20 are greatly reduced. Also, thermal radiation from the targets 110a and 110b to the substrate 20 is low, so that an increase in substrate temperature becomes small. Thus, a film can be formed at low temperature. In contrast to a conventional magnetron-type sputtering method, which encounters difficulty in forming a film of magnetic material at high rate, the facing-targets-type sputtering method can form a film of various materials including magnetic material at low temperature and at high rate and has thus been utilized for manufacturing, for example, a magnetic thin film, a thin-film-type magnetic recording medium, a magnetic head, a metal film and a metal-oxide film.
The facing-targets-type sputtering method usually uses square or circular targets. However, regardless of the target shape, a target tends to be sputtered intensively at a central portion of the target surface, and thus the necessity to improve target utilization efficiency has now been recognized. When a rectangular target is used, a target erosion pattern asymmetrical with respect to a central portion thereof results. As a result, variation in film thickness arises in a width direction of the substrate, indicating the necessity to improve productivity and uniformity of film thickness.
In order to cope with the above-described problems, the present inventors have proposed technology for expanding a sputtering region to the entire target surface in Japanese Publication of Examined Patent Application (kokoku) Nos. H03-2231 and S63-54789. Specifically, magnetic field generation means is disposed around the periphery of a target, and a core is disposed at an end portion of the magnetic field generation means; i.e., at an end portion of a magnetic pole, whereby a magnetic field is formed around the periphery of the target. According to the configuration, since a magnetic field is directly generated between the facing cores without involvement of the targets, magnetic field distribution becomes unlikely to be affected by the magnetic permeability and saturation magnetization of a target material and target thickness. Also, the magnetic field for confinement of sputtering plasma is formed around the periphery of a target, so that a sputtering region expands from a central portion of the target to a peripheral edge portion of the target, thereby greatly improving target utilization efficiency. However, the proposed configuration involves a drawback in that, since a discharge voltage increases during sputtering, stable sputtering requires high sputtering gas pressure.
In order to solve the above problem, the present inventors have proposed technology for improving a feature of the facing-targets-type sputtering method; i.e., technology for more uniformly confining the plasma over the entire surface of a target, in Japanese Publication of Examined Patent Application (kokoku) Nos. H04-11624 and H05-75827. In order to generate and confine sputtering plasma, the proposed technology employs electron reflection means for reflecting electrons toward a space in the vicinity of a peripheral edge portion of the surface of a target in addition to means for generating magnetic flux (magnetic field) extending perpendicularly to the surface of a target as employed in a conventional facing-targets-type sputtering apparatus. According to the proposed technology, high-energy electrons drift within a space provided between a pair of facing targets while a magnetic field formed in the vicinity of a peripheral edge portion of the target allows electrons to continue drifting in the vicinity of a peripheral edge portion of the target without being absorbed by a shield located in the vicinity of a magnetic pole. Thus, the ionization efficiency of a sputtering gas is significantly enhanced to thereby solve the above-mentioned problem.
As a result, sputtering efficiency is enhanced over the entire surface of a target. The sputtering technology can form a thin film of very fine structure with excellent characteristics as compared with the case of a conventional magnetron type sputtering method in which a substrate and a sputtering source face each other and which cannot form such a thin film. Also, the technology realizes uniform erosion of a target over its entire surface. Even when a rectangular target is sputtered, symmetry of a target erosion pattern with respect to a central portion of the target is drastically improved.
However, even in the improved facing-targets-type sputtering apparatus, as before, particles sputtered from a target and recoiled gas particles associated with sputtering scatter into a vacuum chamber from all side faces of a space provided between the facing targets. Accordingly, although the improved apparatus exhibits advantages that the front surface of a target is sputtered uniformly and that a thin film having a uniform thickness can be formed on a substrate in a well-controlled condition, the apparatus still involves drawbacks, in that merely a portion of all side faces of the space between the targets that faces the substrate can be used for formation of the thin film and that the gas contained in the walls by particles impinging on vacuum chamber walls is released during sputtering, resulting in an impairment in the quality of the thin film formed on the substrate.
In order to cope with the above problems, the present inventor proposed a box-type facing-targets-type sputtering apparatus in Japanese Laid-open Patent Publication (kokai) No. H10-8246. In the apparatus, five targets define a box-type discharge space such that one side face of the box-type discharge space is open to a substrate. Specifically, a pair of first targets are disposed a predetermined distance away from each other in a facing condition. Three second targets are disposed in the shape of a squarish letter U lying on its side to thereby define, in combination with the paired first targets, a box-type discharge space having an opening portion which faces a substrate. Magnetic field generation means is disposed in the vicinity of the periphery of each first target such that magnetic poles of one first target face the corresponding magnetic poles of the other first target, for the purpose of generating a magnetic field for confinement of sputtering plasma. The magnetic field generation means generates magnetic fields of the following modes: a tubular facing-mode magnetic field extending between and surrounding the first targets; an ordinary magnetron-mode magnetic field which is generated in the vicinity of the inner surface of a peripheral edge portion of each first target so that magnetic lines extend arcuately from the magnetic pole toward the inner surface; a mirror-type magnetron-mode magnetic field extending in parallel with and in the vicinity of the surface of each second target; and magnetron-mode magnetic fields which are generated in the vicinity of the inner surfaces of opposed side edge portions of each second target adjacent to the corresponding magnetic field generation means so that magnetic fields extend arcuately from the magnetic poles toward the inner surfaces. Also, electron reflection means is provided at each of the following positions: end portions of the second targets which partially define the opening portion of the discharge space; and magnetic-pole end portions of the magnetic field generation means which face the discharge space. The thus-configured apparatus forms a thin film on a substrate disposed in front of the opening portion of the discharge space, by use of sputtering plasma generated within the discharge space.
As mentioned above, the paired facing first targets and the three second targets define a box-type discharge space having the opening portion facing a substrate. By employment of the box-type discharge space and the electron reflection means, electrons are confined by means of the above-mentioned magnetic fields and interact with one another, thereby generating and confining high-density plasma expanding over substantially the entire surface of each target. Thus, substantially the entire surface of each target is uniformly sputtered, thereby solving the aforementioned problems.
However, the above-mentioned facing-targets-type sputtering method has been found to involve the following problem. When a film is formed on a film that has been formed previously, the quality of the previously formed film is impaired in some cases ,for example the case which the previously formed film is an organic material. An investigation as to the cause of the impairment revealed that a sputtering voltage; i.e., a discharge voltage, is responsible. An increase in discharge voltage increases kinetic energy of recoiled gas particles and the strength of an accelerating electric field for negative ions in a cathode sheath. The accelerating electric field in a cathode sheath brings about disturbance on a process of arranging sputtered particles on deposition surface when the deposited film consists of organic materials or oxides, or involves rare earth elements. A discharge voltage depends on, for example, a gas pressure and the mechanical layout of target units. Since, for example, a change in gas pressure influences the quality of a formed film, the adjustment of the discharge voltage has been difficult.
Recently, in the area of advanced thin films, there has been desired formation of a film under high vacuum; i.e., without influence of sputtering gas. This is particularly desired in production of LSIs and like devices, in order to highly reliably form a conductive film on the wall of a contact hole of high aspect ratio having a submicron or sub-half-micron diameter.